The present invention relates to an optical head used for optical recording, reproducing or erasing for information recording media such as an optical disk.
Many reports have been published on techniques for detecting a tracking error signal in an optical head. Push-pull technique is well known as one of representative techniques and it is used practically.
An optical head using push-pull technique is explained below. In the optical head, a light emitted by a light source is condensed by an object lens to form a light spot on a plane of an optical disk for recording information on which a continuous groove of information track is formed spirally. Two photosensitive areas are provided by dividing a photosensitive area of the photodetector with a division line. The photodetector is shown in FIGS. 1-3 at the left side. The light reflected from the optical disk enters to the photodetector. Two photo-detecting signals from the two photosensitive areas are subjected to differential amplification to generate a tracking error signal. Tracking control is performed by controlling the position of the object lens in response to the tracking error signal. When the light spot is subjected to focus control, light intensity distribution of the reflected light is affected by diffraction at the continuous groove due to a position shift of the light spot from the groove. If the prior art optical head is used for an optical disk, light intensity distribution of the reflected light is known to be ascribed to interference of diffracted light beams of 0th, +1st and xe2x88x921st orders at the continuous groove. Two hatched areas in circular light beam in FIGS. 1-3 denote interference regions. The light intensities in the two regions become asymmetrical according to the position shift of the spot from the continuous groove, and the differential signal is used as the tracking error signal.
The above-mentioned prior art optical head has a simple structure to detect a tracking detection signal. However, it has a problem that an offset of the tracking detection signal is generated due to tracking movement of the object lens to the information track or to the tilting of the optical disk. This problem is explained below. FIGS. 1-3 show positions of the light beam on the photodetector in three cases and tracking error signals therefor. The abscissa of graphs of tracking error signal in the three cases illustrated at the right side in FIGS. 1-3 denotes a relative position Xxe2x80x2 of the center of the spot to the track. The tracking error signal shows a waveform schematically when the light spot crosses tracks. FIG. 1 shows a case where the object lens is located above the reference point (X=0). Because the light beam extends symmetrically relative to the division line between the two photosensitive areas, the tracking error signal changes symmetrically with no offset. On the other hand, in a second case where the object lens moves along X direction (or +X direction in the case shown in FIG. 2), the position of the light beam is shifted on the photodetector and the light beam distribution becomes asymmetrical. Then, the tracking error signal has a positive offset relative to the reference voltage. Tracking control performance is deteriorated if a value (Axe2x88x92B)/(A+B) exceeds 20% where A and B denote the positive and negative maximum voltages of the tracking error signal.
Further, when an optical disk is tilted relative to the photodetector along xcex8 direction, the light beam distribution becomes asymmetrical. FIG. 3 shows a case where an optical disk is tilted in xe2x88x92xcex8 direction. The position of the light beam is also shifted on the photodetector in this case and the light beam distribution becomes asymmetrical relative to the division line. Then, the tracking error signal has an offset. Therefore, if the optical disk is tilted in +xcex8 direction and the object lens is shifted in +X direction, the offset of the tracking error signal increases as a sum of the two causes. In an ordinary optical disk, tolerance of off-track is about 0.1 xcexcm where off-track denotes a shift of position of zero tracking error signal relative to the track center. Tracking control for an optical disk is usually needed in a range of about 200 xcexcm of the shift of the object lens and in a range of about 1xc2x0 of tilt of the optical disk. However, in the prior art push-pull optical head, if X is 100 xcexcm and the tilt along xcex8 direction is 0.5xc2x0, the value of (Axe2x88x92B)/(A+B) is 35% and off-track is 0.12 xcexcm. Therefore, the two values exceed the tolerances.
Because the prior art optical head using push-pull technique has the above-mentioned characteristics, an apparatus for reproducing an optical disk with the prior art optical head needs a means for carrying the optical head at a fast speed precisely for fast search to an object information track or for an optical disk having a large eccentricity such as about 100 xcexcm. Then, though the optical head of simple structure is installed, the optical disk reproducing apparatus becomes expensive. Further, because the means for carrying the optical head needs fast speed and high precision, it is not easy to increase tolerance for external shock and vibrations. Therefore, the optical head of push-pull technique is difficult to be installed in a portable optical disk reproducing apparatus.
A push-pull system is used in a focus and tracking error detector apparatus described in U.S. Pat. No. 5,113,386 by Whitehead et al. In order to make the tracking error output signal insensitive on side areas in a detector array, the detector array has a plurality of detectors, and masks or open areas are positioned at side areas outside the central areas including regions where the zeroth order diffraction beam overlaps the first order beams. However, this push-pull system does not solve the above-mentioned problem on the offset of the tracking error signal.
The present invention intends to solve the aforementioned problems, and its object is to provide an optical head which has a simple structure as the optical head of push-pull technique and reduces an offset of tracking error signal due to shift of the object lens and tilt of optical disk.
In one aspect of the present invention, an optical head comprises a light source which emits a light condensed by an optical system. A focus controller controls the optical system to form a light spot on an information recording medium. A photodetecting means has a first division line dividing the light beams and at least one light-shielding area arranged symmetrically to the first division line. The photodetecting means divides a light reflected from the information detection medium to a plurality of light beams and detects the plurality of light beams. An operator operates signals of the plurality of light beams detected by the photodetecting means to supply a tracking error signal. Then, a tracking controller controls the optical system according to the tracking error signal to make the light spot follow an information track formed on the information recording medium. In the photodetecting means, a first division line divides the light beam reflected from the information recording medium into, for example, two, and at least one light-shielding area is arranged symmetrically to the first division line and shields a part of a region (preferably a region wherein first order diffraction light beams reflected from the information recording medium overlap with each other). Thus, the light beam is divided by the first division line except the regions covered by the light-shielding area where, the light intensity distribution is affected largely by tilt of optical disk. Thus, the offset of tracking error signal due to tilt of an optical disk is decreased.
Preferably, the width V of the light-shielding area along the same direction as the first division line satisfies a following relation:                                           0.1            ⁢                                          1                -                                                      (                                          1                      4                                        )                                    ⁢                                                            (                                              λ                        NAd                                            )                                        2                                                                                 less than                       (                          V              D                        )                     less than                       0.5            ⁢                                          1                -                                                      (                                          1                      4                                        )                                    ⁢                                                            (                                              λ                        NAd                                            )                                        2                                                                                      ,                            (        1        )            
wherein D is diameter of light beam, NA is numerical aperture, xcex is wavelength and d is a track pitch of the information track. If a plurality of divided light beams is detected by setting the width V as explained above, the optical intensity can be sufficient for deriving the tracking error signal while the regions which are liable to be affected by tilt of optical disk are shielded. Then, the tracking error signal is not largely affected by tilt of optical disk. Further, if the light-shielding area extends in parallel to moving direction of the object lens, even if the reflected light beam is shifted due to movement of the object lens, a similar advantage is realized.
In another aspect of the present invention, in the photodetecting means, second and third division lines perpendicular to the first division line further divide the light beam besides the first division line. Then, the light beam is divided into, for example, six to detect the light. Further, at least one light-shielding area is arranged to extend symmetrically to the first division line. Thus, the photodetecting means divides the light beam into regions affected largely due to overlapping of zeroth and first order diffraction light beams in the light reflected from the information recording medium. Thus, the offset of tracking error signal due to shift of the reflected beam can be corrected selectively. Preferably, the distance U between the second and third division lines satisfies a following relation:                                           0.8            ⁢                                          1                -                                                      (                                          1                      4                                        )                                    ⁢                                                            (                                              λ                        NAd                                            )                                        2                                                                                 less than                       (                          U              D                        )                     less than                       1.1            ⁢                                          1                -                                                      (                                          1                      4                                        )                                    ⁢                                                            (                                              λ                        NAd                                            )                                        2                                                                                      ,                            (        2        )            
wherein D is diameter of light beam, NA is numerical aperture, xcex is wavelength and xe2x80x9cdxe2x80x9d is a track pitch of the information track. Then, the photosensitive areas outside the second and third division lines do not include all or almost all the interference regions. Therefore, signals which are affected little by tilt of optical disk and corresponds to the shift of the reflected beams can be taken out selectively. Thus, the offset due to shift of the reflected beam can be corrected advantageously.
In a third aspect of the invention, the light-shielding area comprises a first light-shielding portion extending symmetrically from the first division line in two directions, and second and third light-shielding portions provided outside the boundaries of the first light-shielding portion (or peripheries of the light beam). By providing the second and third light-shielding portions, even if the beam reflected from an optical disk is shifted, areas for detecting tracking error signal is limited to equal distance from the first division line. Thus, the offset of tracking error signal due to shift of the light beam becomes small, and the offset is decreased. Preferably, the distance between the second and third light-shielding portions satisfies a following relation:                                           1            -                          (                                                3                  ⁢                  a                                D                            )                                 less than                       W            D                     less than                       1            -                          (                              a                D                            )                                      ,                            (        3        )            
wherein D is diameter of light beam and a is maximum shift of light beam.
An advantage of the present invention is that the optical head supplies a tracking error signal having small offset due to shift of object lens along tracking direction.
An advantage of the present invention is that the optical head supplies a tracking error signal having small offset due to tilt of optical disk.